New USDA planting map: A few cautions

Whoa everybody! Let’s just step back a sec and take a deep breath.

The US Department of Agriculture revised its zone hardiness map for the first time since 1990. Our area’s “promotion” into Zone 6 from Zone 5 has brought some here to the brink of hysteria.

“Welcome to the warmer zone” reads the headline today in the Kansas City Star. “Gardeners, rejoice! New planting guide map opens up more choices of plants that usually thrive farther south.”

The story gets front page treatment and is illustrated with beautiful sprigs of crepe myrtle in all their pink-hued glory.

But let’s fight off the urge to run to the nearest garden center and take a hard look at what that really means.

The facts: The USDA Zone Hardiness map is mainly a guide for figuring out what plants can survive in the ground outside through the average winter. So it focuses on the coldest average winter lows. Our former designation in Zone 5 had the coldest expected lows at  minus 10 to minus 20. The new Zone 6 raises the expected low temperatures to zero to minus 10.

That change isn’t really news to anyone who has been watching the weather. It’s been 20 below maybe two or three times since we’ve lived here. In fact, any below-zero temperature draws a lot of attention, it’s so rare.

And–news flash–people have been trying and sometimes succeeding with Southern plants here for years. We’ve all seen the crepe myrtle, rhododendrons and the odd magnolia trees. The new zone map won’t make them any less difficult to grow here. It will take years of global warming to do that. Possibly. True, local nurseries may start carrying a few more warm weather varieties, but they’ve always been available from catalogs for anyone brave (or foolish) enough to try them.

Here are a couple of reasons for caution, before you begin digging the hole for that banana tree:

1.The Hardiness Zones are a one-dimensional guide for planting. But plants have a lot of other needs. Humidity, rainfall, shade, wind protection, soil pH. The west Texas panhandle and northern Alabama are both in Zone 7, but that doesn’t mean you can grow the same things equally well in both places.

Azaleas, for instance, will not stop needing moisture in the winter, no matter what zone they’re in. So growing them here is always going to be trickier than it would be in Virginia. A new zone designation won’t really change that.

And let’s not forget summer and spring highs. Tender berries and spinach don’t much like to see 90 degrees in May, and tomatoes won’t set fruit much past 90 either, as was well proved during last summer’s oppressive heat.

2.Snow matters, too. Snow cover actually insulates root systems against the extremes of cold. A mild winter with little snow followed by a cold snap can do more damage than predictably cold and snowy winters. Mild weather also encourages budding. And the zone change doesn’t mean there won’t be freakish cold snaps that will kill flowers and fruit. It was only five or six years ago that we had several nights of below zero in April.

3. Warm winters are not such good news for some plants that have grown well here in the past. What you gain in Southern flowering shrubs on one hand, you may lose in fruit trees on the other.

Apples come to mind. There’s a reason you don’t hear of massive apple orchards in the Bahamas. That’s because apples and other fruit trees need cold winter weather to promote spring growth. If they don’t get it, budding and fruiting will be weak. (Look here for more on fruit trees.)

4.I looked, but couldn’t find a comparable map for insect winter survival. (Hint, hint, USDA). But it only stands to reason that if different plants can now survive Kansas City winters, then different bugs will be landing here soon. Insects have special affinities for certain plants, and when those plants move north, you can expect to see the bugs come with them.

Fire ants, anyone?

Ok, ok. Get excited about the new zone map if you really must. It’s a great dream, I’ll concede that much. But Kansas City will always be Kansas City, no matter what the USDA says.

Posted by: Roxie

 

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